Pressure pulp screen

ABSTRACT

A pulp screen of the vertical pressure type having a cylindrical screen and an impeller mounted for rotation within the screen, a conical core on the impeller shaft defines a primary screening compartment below the pulp stock inlet compartment and a secondary screening compartment below the primary compartment, the conical core has a first angled wall defining an upper annular stock screening area of the primary compartment, a series of pairs of blades radiate from the conical core and define a series of radial passages extending the length of the core, and dilution water is fed to the radial passages and to the face of the screen.

United States 1 [111 3,713,536 Hooper 1 Jan. 30, 1973 541 PRESSURE PULPSCREEN 2,246.66) 6/1941 Cowan ..209/273 x [75] Inventor: Sydney W. H.Hooper, Sherbrooke,

Quebec, Canada Assignee: S. W. Hopper & Co., Ltd., Sherbrooke, Quebec,Canada Filed: Nov. 30, 1970 Appl. No.: 93,671

Field of Search ..209/273, 305, 306, 270, 380, 209/268, 300, 250

References Cited UNITED STATES PATENTS 4/1966 Cowan ..209/273 X PrimaryExaminer-Frank W/Lutter Assistant Examiner-William A. Cuchlinski, Jr.Att0mey-Fetherstonhaugh & Co.

[57] ABSTRACT A pulp screen of the vertical pressure type having acylindrical screen and an impeller mounted for rotation within thescreen, a conical core on the impeller shaft defines a primary screeningcompartment below the pulp stock inlet compartment and a secondaryscreening compartment below the primary compartment, the conical corehas a first angled wall defining an upper annular stock screening areaof the primary compartment, a series of pairs of blades radiate from theconical core and define a series of radial passages extending the lengthof the core, and dilution water is fed to the radial passages and to theface of the screen.

15 Claims, 7 Drawing Figures PATENIEDJAHO I875 3,713,536

sum 1 or 5 INVENTOR S. W. H. HOOPER PATENT AGENTS PATENTEnJmo 1913 SHEET2 [IF 5 INVENTOR S. W. H. HOOPER PA TENT AGENTS PATENTEDJAH 30 I975SHEET 3 BF 5 INVENTOR S. W. H. HQOPER I @vr I'A'I'I'INT AGENTSPATENTHHMI 3 0 W5 SHEET t []F 5 INVENTOIRI S. W. H. HOO PER V11 'l'I-INTAGENTS PATENTEDmso I975 sum 5 or 5 zoo 0000000000000 INVENTOR S. W. H.HOOPER PA TENT AGENTS PRESSURE PULP SCREEN This invention relates to therotary screening of pulp stock, and more particularly to verticalpressure type pulp screens in which the rotor has a particularconfiguration in relation to the screen to ensure a maximum separationof the pulp fibers from reject material.

To obtain efficient removal of fiber bundles and slivers suspended inthe inlet pulp, and for more efficient screening, it is imperative tointroduce not only circular rotation of the pulp slurry but a definiteaxial velocity of the same slurry has to be also present at the Y innerside of the screen plate. The axis of the pulp particles have to beoriented parallel'with the screen plates and the particles have to movein a circular direction and axial direction along the screen plates atthe same time.

It is assumed, and indirectly supported by test results, that in thegravity screen this axial movement is present. In the gravity screen theaction of the centrifugal force, the controlled volume of pulp presentin the screening area, and the gradual draining of the pulp through thescreen plate forces the inner surface of the pulp slurry to take up theshape of a parabolic cone with an air and moisture filled void at thecenter of the screen. This configuration results in a liquid headdifferential between the inlet-end and the reject-end of the screen.This head produces the required axial velocity of the pulp slurry alongthe screen plate towards the reject end of the screen. When the screenis pressurized this dynamic configuration of the pulp within thescreening area of the screen ceases to exist.

The design of the screen in the present application takes the aforesaidinto considerationand the configuration of the cone is such that it willconfine the pulp slurry to the same geometric form as is present in agravity discharge screen under running conditions, i.e., a geometricform having the shape of a parabolic cone. The impeller consists of atruncated cone whose inlet end is level with the inlet side of thescreen. The opposite end of the cone is level with the reject end of thescreen. The distance between the impeller face and screen is larger atthe inlet than at the reject end, this the cross-sectional area of thezone through which the pulp moves diminishes. This reduction willcompensate for the reduction of pulp slurry volume as the latter istransferring through the screen, thus the axial flow velocity will be amaintained constant.

The inside of the impeller is hollow consisting of an upper sealed offcompartment which has only structural purposes and a lower, largercompartment which provides passage for the shower water.

The inside'edge of the larger, or reject end of the impeller is equippedwith a labyrinthe and forms a narrow gap with the shower waterdistributing chamber to control the flow of the dilution water to thereject chamber. The configuration of the large, or reject end of theimpeller-in combination with the labyrinthe seal-prevents the escape ofthe shower water from the interior of the impeller, thus the showerwater can be maintained at a higher pressure and the combination of thepressure and the centrifugal force acting on the trapped shower waterwill force the shower water through the shower holes in the side wall ofthe impeller.

As the pulp slurry proceeds axially along the screen plate, drainagethrough the screen plate takes place and the consistency of the pulpslurry increases. To prevent this, and to improve separation of thefiber material capable of passing through the screen perforations fromthe material to be rejected, showering or dilution water is introducedright at the screen plate over approximately three-fourths of the lengthof the screening area.

The dilution or shower water is introduced through the bottom or outsideshell of the pulp screens main body into a shower water distributingchamber, forming a confined area around the shaft housing. Through largeholes in the top of this chamber the shower water in turn passes intothe shower water chamber in the impeller. I

The lower portion of the outside wall of the shower distributing chamberforms an inner annular wall of the reject chamber.

Attached .to the impeller are the rotor blades. Each blade consists oftwo equal plates attached to the impeller in a radial position andparallel to each other with a narrow opening in between. These bladeshave the same length as the impeller and their outsideedges are parallelwith the screen plate. Only a small gap is left between the impellerblades and the screen plate. Positioned between the narrow gap of eachtwin blade are the shower water holes in the wall of the lowercompartment of the impeller.

The purpose of the impeller blades is two-fold. Firstly they provide forthe circular rotation of the pulp slurry, secondly the twin bladeconstruction with both blades being of equal length delivers the showerwater directly into the fiber mat building up on the adjacent surface ofthe screen plate. The shower water is forced through the holes of theimpeller and through the slots of the pairs of blades partly by thepressure in the shower water. chamber of the impeller and thecentrifugal force generated by rotation of the impeller. I

The pulp slurry enters the screen tangentially to the first, or inletpart of the screen. An inlet ring attached to the top of thescreening'chamber covers half of the impeller blades in order toreduceeddying around the upper tip of the blades. The vertical part ofthis ring and the outside of the inlet chamber forms an annular areawithin which the tangentially entering pulp slurry 'is forced to rotate.This initial circular motion reduces the forces required to drive thescreen rotor. Also the centrifugal action of the rotating stock willforce heavy particles to deposit in this inlet annular chamber andprevent them fromentering the screening zone where they can causedamage.

The lighter particles of the pulp slurry flows over the vertical part ofthe inlet ring and into the screening portion proper of the machinewhere the good fibers are passed through the screen and any rejectparticles left in the slurry. are carried. off through a rejectdischarge outlet.

The largeor reject end of the rotor, the outer shell of the shower waterdistributing chamber and the lower support ring of the screen platesform the reject chamber. The pulp slurry that did not pass through thescreen plate will enter this reject chamber passing the narrow gapbetween the reject end of the screen plate and the larger diameterperiphery of the impeller. Dilution water to this chamber is suppliedthrough the narrow gap formed by the outer shell of the dilution chamberand the labyrinthe at the inner edge of the impellers large or rejectend plate. The rejects are discharged through the reject line whichpasses through the outer shell of the screen body. To provide positivedischarge of the rejects, rotor blades are attached to the large orreject end plate of the impeller preferably in axial alignment with atleast one of the pair of rotor blades.

An example embodiment of the invention is shown in the accompanyingdrawings in which:

FIG. I is a vertical sectional elevation of the pulp screen of thepresent invention.

FIG. 2 is a horizontal section taken on the line 2-2 of FIG. 1.

FIG. 3 is a horizontal section taken on the line 3-3 of FIG. 1.

FIG. 4 is a horizontal section taken on the line 4-4 of FIG. 1.

FIG. 5 is an enlarged partial vertical sectional detail showing theannular stock inlet to the screening chamber.

FIG. 6 is a partial plan view taken on the line 6--6 of FIG. 5.

FIG. 7 is a partial vertical elevation of the surface of the impeller inway of one of the blades.

Referring to the drawings, the pulp screening machine 5 consists of avertically disposed circular housing 6, a top sealing cover 7 and abottom sealing plate 8.

The circular housing 6 is provided with an annular support ring 9towards its upper end and a lower sup port ring 10 between which theannular perforated screen plate 1 1 is supported.

A pulp stock inlet 12 directs the pulp to be screened into the inletchamber 13, located above the support ring 9.

An impeller rotor 14 is formed of a truncated cone having an upperportion 5 and a lower portion 16, with the peripheral wall 15a of theupper portion 15 being disposed at a greater angle to the axis of theimpeller 14 than the peripheral wall 16a of the lower portion 16.

The impeller 14 is supported on and isrotated by the drive shaft 17which, in turn, is driven by the pulley 18 and belt drive 19.

The interior of the upper portion 15 of the impeller 14 is a closedchamber 20 formed by the peripheral wall 15a, the upper end wall 21, theinterior wall 22, and the internal sleeve 23 is keyed at 24 to the upperend 25 of the shaft 17.

An axial sleeve 26 projects upwards from the bottom sealing plate 8 intothe interior of the lower portion 16 of the impeller 14 and houses thebearings 27 supporting the drive shaft 17.

The dilution shower water is fed through the inlet pipe 28 to theannular shower water distributing chamber 29 built around the lowerportion of the sleeve 26. A series of apertures 30 in the top wall 31 ofthe chamber 29 permits the shower water to flow into the chamber 32within the lower portion 16 of the impeller 14.

A series of pairs of blades 33 radiate outwards from the outer surfaceof the impeller 14 at spaced intervals around the circumference thereof,and extend radially outwards to within a short distance of the inneradjacent surface of the annular screen 11 to form the longitudinalnozzles 34 extending the full length of the impeller 14. The peripheralwall of the lower portion of the impeller 14 is perforated at 35 in thespaces between the pairs of blades 33 to permit the flow of shower waterfrom the chamber 32 into the nozzles 34 and against the interior surfaceof the annular screen 11.

The lower end wall 36 of the impeller 14 is formed with a labyrinthe 37facing the vertical wall 38 of the water distributing chamber 29 topermit a controlled volume of the dilution water to flow from thechamber 32 downwards into the reject chamber 39.

One blade of each pair of blades 33 of the impeller 14 projectsdownwards into the reject chamber 39 and is extended radially inwards toform the paddles 40;.

These paddles 40 are designed to propel reject material towards thereject discharge housing 41 and discharge pipe 42. At the same time, anygood pulp fibers adhering to the reject material is loosened and islater recovered.

An inlet ring 43, of right angle cross-section, is secured by means ofits flange member 44, along with the top end of the annular screen 11,to the screen support ring 9. The upstanding flange member 45 of theinlet ring 43 is located radially inwards of the annular screen 11 sothat the flange member 44 partially overlaps the exposed top ends 46 ofthe impeller blades 33 and, with the adjacent peripheral edge 47 of thetop end wall2l of the impeller 14, defines the pulp stock inlet 48 fromthe inlet chamber 13 to the screening chamber 49.

The screened pulp, after passing through the annular screen 11 into thechamber 50, is discharged from the apparatus through the outlet 51.

In the operation of this invention, the pulp slurry enters the chamber13 through the inlet 12 and is concentrated with a tangential motionwithin the annular area 13a formed by the inlet ring 43, and isconstrained to enter the upper part of the screening chamber 49 throughthe annular inlet 48.

As the inlet ring 43 overlaps a portion of the top edge of the rotorblades 33, it has the effect of reducing eddy currents around the upperends 46 of the impeller blades. Y

The vertical flange 45 of the ring 43 and the outer wall of the inletchamber 13 combine to force the tangentially entering pulp stock slurryto take on a rotary motion. This initial rotary motion of the slurry hasthe effect of reducing the forces required to drive the screen impellerand consequently reduces the cost of operating the machine. In addition,the centrifugal action of the rotating pulp stock will force heavyparticles carried by the stock to be deposited within the annular area13a of the inlet chamber 13 and thus prevent at least a large proportionof reject material from entering the screening chamber 49 and therebymaterially reduces the quantity of reject material which would have tobe discharged through the outlet 41. Also, by extracting at least aportion of reject material at the annular area 13a of the inlet chamber13, damage to the screen 11 will be materially reduced.

As will be seen in FIG. 1, the wall of the upper portion 15 of the rotor14 is angled more steeply than is the wall of the lower portion 16. Thishas the effect of permitting a large flow of stock to enter thescreening chamber 49 from the inlet chamber 13 with a parabolic coneform and, as the cross-sectional area of the screening chamber isreduced, a high velocity of flow of stock through the screening chamberis maintained.

As the pulp stock moves down the screening chamber and tends to thickenon the screen 11, the dilution water from the chamber 29 is forcedthrough the apertures 30 into the chamber 32 and through the apertures35 into the nozzles 34 to impinge on the screen 11 and force the pulpfibers through to the outer chamber 50 from where the good fibers arecarried away through the discharge outlet 51.

As the nozzles 34 extend the full length of the rotor blades 33 and theoutlet of the nozzles is close to the surface of the screen 11 over thewhole length thereof, the dilution water, in addition to carrying thepulp fibers through the screen sweeps the fibers ahead on the surface ofthe screen and thereby dilutes the forming mat to permit the flexiblefibers to orient themselves and pass through the apertures in the screenplate 11, and with the dilution water between the blade formed nozzlesthins the pulp slurry to further hasten the passage of the fibersthrough the screen.

By the time a reduced volume of pulp stock reaches the bottom of thescreening chamber 49, all of the pulp fibers have been forced throughthe screen 11, except for a very small amount which have not been washedof reject material, and carried down into the reject chamber 39.

At all times during the operation of the screening of the pulp, theoptimum pressure balance within the apparatus will be maintainedconsistant with the parameters of the pulp slurry being fed to theapparatus.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A rotary pulp screening apparatus comprising a housing having anupper pulp stock inlet chamber and a lower pulp stock screening chamber,said inlet chamber having a tangentially disposed stock inlet and thescreening chamber having a screened pulp discharge outlet, a cylindricalscreen mounted within the screening chamber,

a rotary impeller having an axial length extending for the length of thescreen and mounted for rotation axially within the screen, said impellerbeing in the form of a truncated cone having an upper portion and alower portion, the peripheral wall of the upper portion having an angleto the axis of the cone greater than the angle of the peripheral wall ofthe lower portion with the axis of the cone,

the upper end of the impeller facing the inlet chamber being closed, anannular inlet ring spaced radially from the upper end of the impeller atthe upper end of the screen, said space between the upper end of theimpeller and the inlet ring forming an annular inlet to the screeningchamber at the upper end of said screen,

impeller blades radiating from said impeller and extending to within ashort distance of the radial inward facing surface of said screen, saidblades extending for the full axial length of the said impeller,

and means on said impeller for directing controlled jets of dilutionwater from inside said impeller over a major portion of the axial lengthof the radially inward facing surface of said screen.

2. Apparatus as set forth in claim 1 in which the interior of the upperportion of the impeller is divided from the lower portion of the cone bya division wall and shaft means to rotate the impeller is secured tosaid upper portion.

3. Apparatus as set forth in claim 2 in which the lower portion of theimpeller includes a sleeve housing a bearing assembly for said shaftmeans.

4. Apparatus as set forth in claim 1 in which said housing has a lowerend wall closing said screening chamber, and a sleeve projects upwardsfrom said end wall and into the interior of said impeller, the lower endof said impeller having a labyrinth seal about said sleeve.

5. Apparatus as set forth in claim 4 in which said sleeve includes anannular shower water distributing chamber having a water inletconnection, and a series of apertures in said distributing chamberpermit the flow of water from the chamber to the interior of the lowerportion of the impeller.

6. Apparatus as set forth in claim 1 in which the peripheral wall of thelower portion of the impeller has a series of perforations locatedbetween the pairs of blades forming said nozzles, and shower water isfed to the interior of the lower portion of the impeller and throughsaid nozzles.

7. Apparatus as set forth in claim 1 in which the said inlet ring is ofright angle section, a first member of which is secured to the upper endof the said annular screen, and a second member of which is projectedupwards in said inlet chamber to form an outer annular reject collectingarea and an inner annular inlet from the inlet chamber to the screeningchamber.

8. Apparatus as set forth in claim 7 in which said second member of theinlet ring forms an annular weir for retention of reject material withinthe inlet chamber.

9. Apparatus as set forth in claim 1 in which said inlet ring partiallyoverlaps the upper ends of said blades of the impeller.

10. A rotary pulp screen according to claim 1, wherein said impellerblades are arranged in pairs of blades, the blades of each pair beingclose together and said means for directing controlled jets of dilutionwater including means for directing the water through the impeller andbetween the blades of each pair such that each said pair of blades actsas a nozzle for directing the dilution water against the inward facingsurface of the screen.

11. A rotary pulp screen according to claim 10, wherein said means fordirecting the dilution water is directed only through said lower portionof the impeller.

12. A rotary pulp screen according to claim 10, including a rejectchamber located below the said screening chamber and wherein at leastone blade of each pair extends downward into the reject chamber.

13. A rotary pulp screen according to claim 1, including a rejectmaterial chamber located below the screening chamber, and wherein atleast some of said blades extend down into the reject chamber.

14. A rotary pulp screen according to claim 13, said blades in thereject chamber extending radially inwardly to form paddles.

15. Apparatus as set forth in claim 13 in which said reject materialchamber has a discharge opening directed radially outwards into adischarge housing and a discharge outlet leads from said dischargehousing to the exterior of the apparatus.

1. A rotary pulp screening apparatus comprising a housing having anupper pulp stock inlet chamber and a lower pulp stock screening chamber,said inlet chamber having a tangentially disposed stock inlet and thescreening chamber having a screened pulp discharge outlet, a cylindricalscreen mounted within the screening chamber, a rotary impeller having anaxial length extending for the length of the screen and mounted forrotation axially within the screen, said impeller being in the form of atruncated cone having an upper portion and a lower portion, theperipheral wall of the upper portion having an angle to the axis of thecone greater than the angle of the peripheral wall of the lower portionwith the axis of the cone, the upper end of the impeller facing theinlet chamber being closed, an annular inlet ring spaced radially fromthe upper end of the impeller at the upper end of the screen, said spacebetween the upper end of the impeller and the inlet ring forming anannular inlet to the screening chamber at the upper end of said screen,impeller blades radiating from said impeller and extending to within ashort distance of the radial inward facing surface of said screen, saidblades extending for the full axial length of the said impeller, andmeans on said impeller for directing controlled jets of dilution waterfrom inside said impeller over a major portion of the axial length ofthe radially inward facing surface of said screen.
 1. A rotary pulpscreening apparatus comprising a housing having an upper pulp stockinlet chamber and a lower pulp stock screening chamber, said inletchamber having a tangentially disposed stock inlet and the screeningchamber having a screened pulp discharge outlet, a cylindrical screenmounted within the screening chamber, a rotary impeller having an axiallength extending for the length of the screen and mounted for rotationaxially within the screen, said impeller being in the form of atruncated cone having an upper portion and a lower portion, theperipheral wall of the upper portion having an angle to the axis of thecone greater than the angle of the peripheral wall of the lower portionwith the axis of the cone, the upper end of the impeller facing theinlet chamber being closed, an annular inlet ring spaced radially fromthe upper end of the impeller at the upper end of the screen, said spacebetween the upper end of the impeller and the inlet ring forming anannular inlet to the screening chamber at the upper end of said screen,impeller blades radiating from said impeller and extending to within ashort distance of the radial inward facing surface of said screen, saidblades extending for the full axial length of the said impeller, andmeans on said impeller for directing controlled jets of dilution waterfrom inside said impeller over a major portion of the axial length ofthe radially inward facing surface of said screen.
 2. Apparatus as setforth in claim 1 in which the interior of the upper portion of theimpeller is divided from the lower portion of the cone by a divisionwall and shaft means to rotate the impeller is secured to said upperportion.
 3. Apparatus as set forth in claim 2 in which the lower portionof the impeller includes a sleeve housing a bearing assembly for saidshaft means.
 4. Apparatus as set forth in claim 1 in which said housinghas a lower end wall closing said screening chamber, and a sleeveprojects upwards from said end wall and into the interior of saidimpeller, the lower end of said impeller having a labyrinth seal aboutsaid sleeve.
 5. Apparatus as set forth in claim 4 in which said sleeveincludes an annular shower water distributing chamber having a waterinlet connection, and a series of apertures in said distributing chamberpermit the flow of water from the chamber to the interior of the lowerportion of the impeller.
 6. Apparatus as set forth in claim 1 in whichthe peripheral wall of the lower portion of the impeller has a series ofperforations located between the pairs of blades forming said nozzles,and shower water is fed to the interior of the lower portion of theimpeller and through said nozzles.
 7. Apparatus as set forth in claim 1in which the said inlet ring is of right angle section, a first memberof which is secured to the upper end of the said annular screen, and asecond member of which is projected upwards in said inlet chamber toform an outer annular reject collecting area and an inner annular inletfrom the inlet chamber to the screening chamber.
 8. Apparatus as setforth in claim 7 in which said second member of the inlet ring forms anannular weir for retention of reject material within the inlet chamber.9. Apparatus as set forth in claim 1 in which said inlet ring partiallyoverlaps the upper ends of said blades of the impeller.
 10. A rotarypulp screen according to claim 1, wherein said impeller blades arearranged in pairs of blades, the blades of each pair being closetogether and said means for directing controlled jets of dilution waterincluding means for directing the water through the impeller and betweenthe blades of each pair such that each said pair of blades acts as anozzle for directing the dilution water against the inward facingsurface of the screen.
 11. A rotary pulp screen according to claim 10,wherein said means for directing the dilution water is directed onlythrough said lower portion of the impeller.
 12. A rotary pulp screenaccording to claim 10, including a reject chamber located below the saidscreening chamber and wherein at least one blade of each pair extendsdownward into the reject chamber.
 13. A rotary pulp screen according toclaim 1, including a reject material chamber located below the screeningchamber, and wherein at least some of said blades extend down into thereject chamber.
 14. A rotary pulp screen according to claim 13, saidblades in the reject chamber extending radially inwardly to formpaddles.